Intracellular receptors mediate the actions of glucocorticoids. Unfortunately, little is known about the processes that control receptor levels. Recently the Principal Investigator reported that glucocorticoid agonists regulate the number of glucocorticoid receptors in the ACTH-screting, glucocorticoid-responsive AtT-20 mouse pituitary tumor cell line. Additionally, recent preliminary studies have shown that although glucocorticoids dramatically decrease the number of their receptors in the AtT-20 cell, they do not lose their potency at suppressing ACTH. This discrepancy between receptor binding and against action suggests that the mechanism of glucocorticoid action at the nucleus may be more complicated than currently appreciated. This proposal investigates the processes of receptor depletion and repletion in the AtT-20 cell. The goal is to elucidate the mechanisms and sites of receptor turnover and additionally investigate whether agonist-induced receptor depletion is an integral part of the long-term nuclear mechanism of steroid hormone action. The exeriments are divided into 4 sections. The first attempts to establish the molecular pathway and subcellular sites of receptor depletion. The second investigates the process of receptor repletion. The third quantitatively correlates agonist-induced receptor depletion with agonist-induce suppression of ACTH. The goal is to demonstrate that the onset and magnitude of depletion are related to ACTH suppression. The goal of the fourth section is to search for other glucocorticoid cell lines that demonstrate agonist-induced receptor depletion. These studies will provide a musch more detailed understanding of the pathways of receptor regulation. Their results will also be significant in several areas of intense medical investigation. For example, several cancers are known to be responsive to hormonal manipulations but only if the patient's tumor contains steroid receptors. The studies in this proposal may suggest why some tumors are receptor negative and, further, may even suggest ways of augmenting the number of receptors in "receptor deplete" neoplastic cells. Additionally many models of aging demonstrate an unexplained age-related loss of glucocorticoid receptors. The studies of this proposal might demonstrate the most likely pathway of receptor loss in these important models.